1. Field of the Invention
The present invention relates to a navigation system or global positioning system and more particularly to a vehicle navigation system that determines a vehicle position using radio signals from three satellites.
2. Description of the Prior Art
A navigation system or global positioning system which determines the position of a moving vehicle from radio signals of satellites is already in service. This global positioning system receives radio signals from a plurality of satellites and calculates the absolute position of a moving body anywhere on Earth to within 100 meters. To obtain two-dimensional position data (longitude and latitude) with sufficiently high accuracy, it is necessary to receive radio signals from four or more satellites at all times. When the number of satellites that are in radio range of the moving body decreases below four, the altitude cannot be determined, giving rise to an error in the two-dimensional position data. Hence, when only three or less satellites are in radio range, the previous altitude data was used to produce the two-dimensional position data. In this case, however, if the moving body has changed its altitude, an error still occurs in the two-dimensional position data.
With the conventional global positioning system, it is possible to produce accurate longitude and latitude data of a moving body when radio signals can be received from four satellites. But when the number of satellites in radio range of the moving body is less than four, errors will inevitably occur in the two-dimensional position data.